1. Census geography

The UK Censuses

The UK Census is undertaken every 10 years, with the most recent being on 27 March 2011. Its purpose is to collect population and other statistics essential to those who have to plan and allocate resources.

Major customers include departments of national and local government, and providers of services such as health and education.

The Census occurs simultaneously in all parts of the UK. In England and Wales, the Office for National Statistics (ONS) is [or “we are”] the responsible body. In Scotland, it is [the?*] National Records of Scotland (NRS), and in Northern Ireland, the Northern Ireland Statistics and Research Agency (NISRA).

The main geographies directly associated with the Census are Output Areas (OA) and Super Output Areas (SOA). OAs are the base unit for Census data releases.

Census data can be produced for most geographies by best-fitting from OA to the required output geography current on 31 December 2011.

Further Information and Census Geography Products

We offer a range of 2011 Census geography products for England and Wales including:

Digital boundaries and centroids, for OAs and SOAs, which you can use in your own geographical information systems to carry out spatial analysis or web mapping

2. Built-up Area / Built-up Area Sub-division

BUAs and BUASDs are a new geography, created as part of the 2011 Census outputs.

This data provides information on the villages, towns and cities where people live, and allows comparisons between people living in built-up areas and those living elsewhere. Census data for these areas (previously called urban areas) has been produced every 10 years since 1981.

A new methodology to capture the areas was used in the 2011 version, but it still follows the rules used in previous versions so that results are broadly comparable. As before, the definition follows a “bricks and mortar” approach, with BUAs defined as land with a minimum area of 20 hectares (200,000 square metres), while settlements within 200 metres of each other are linked.

The BUAs and BUASDs are available as boundary sets, name and codes and lookup files, which can be downloaded from the Open Geography portal.

You will need to be aware that there are areas included in the boundary datasets but not in the Census tables. These BUAs and BUASDs have been identified as areas that have not been allocated a population. In most cases, this is because they do not have any residential buildings – for example, industrial estates, airports, theme parks, etc. There are 337 BUAs where population has not been allocated (305 in England and 32 in Wales) and 133 BUASDs where population has not been allocated (123 in England and 10 in Wales). The names and codes of these areas have been included in documentation that accompanies the files.

3. Enumeration District (ED)

2011 Census

There were no official enumeration districts (ED) created for the 2011 Census.

Households in England and Wales received their census questionnaire through the post and returned them by post or online.

Only communal establishments, for example, care homes, and special groups (such as travellers) had their census questionnaires hand-delivered. Individuals within communal establishments also had the option of completing their questionnaire online following the same process as that used by households.

2001 Census

EDs were used for data collection for the 2001 Census. England and Wales had 116,895 EDs, the majority of which were different from their 1991 equivalents, with an average size close to 200 households (450 people).

Scotland had 6,987 EDs with an average size of 328 households (730 people). Northern Ireland had 2,591 EDs with an average size of 260 households (650 people).

In addition, there were special enumeration districts (SED) for communal establishments with the capacity to house over 100 people. SEDs included prisons, hospitals, nursing homes, halls of residence, large hotels and military bases.

EDs sometimes straddled 2001 administrative boundaries and were deemed unsuitable for data output and were used for data collection only.

Output areas (OA) were introduced for data output.

1991 Census

1991 EDs were used for both data collection and output. Their size and shape was primarily determined by the requirements of data collection, but they fitted the administrative boundaries current at the time.

4. Output Area (OA)

Output areas (OA) were created for Census data, specifically for the output of census estimates. The OA is the lowest geographical level at which census estimates are provided. OAs were introduced in Scotland at the 1981 Census and in all the countries of the UK at the 2001 Census.

2001 Census OAs were built from clusters of adjacent unit postcodes but as they reflected the characteristics of the actual census data, they could not be generated until after data processing. They were designed to have similar population sizes and be as socially homogenous as possible based on tenure of household and dwelling type (homogeneity was not used as a factor in Scotland).

They had approximately regular shapes and tended to be constrained by obvious boundaries such as major roads.

OAs were required to have a specified minimum size to ensure the confidentiality of data.

In England and Wales, 2001 Census OAs were based on postcodes as at Census Day and fit within the boundaries of 2003 statistical wards and parishes. If a postcode straddled an electoral ward/division or parish boundary, it was split between two or more OAs.

The minimum OA size was 40 resident households and 100 resident people, but the recommended size was rather larger at 125 households. These size thresholds meant that unusually small wards and parishes were incorporated into larger OAs.

OAs for Northern Ireland had the same minimum size, as for England and Wales, but were based on postcodes as at January 2000. The OAs fit within the 2001 electoral ward boundaries.

In Scotland, OAs were based on postcodes as at December 2000 and related to 2001 wards. However, the OAs did not necessarily fit inside ward boundaries where confidentiality issues made it more appropriate to straddle boundaries. The minimum OA size was 20 resident households and 50 resident people, but the target size was 50 households.

2011 Output Areas

England and Wales:

Maintaining stability as far as possible was key for the 2011 Census. Some modification of the previous OAs and super output areas (SOA) has taken place where a significant need has occurred since 2001 (see “Modification of Output Areas” below).

The total number of 2011 OAs is 171,372 for England and 10,036 for Wales. There are now 181,408 OAs, 34,753 lower layer super output areas (LSOA) and 7,201 middle layer super output areas (MSOA) in England and Wales. This means that 2.6% of the 2001 OAs have been changed as a result of the 2011 Census, along with 2.5% of LSOAs and 2.1% of MSOAs.

Significant points of interest for the 2011 Census are that OAs and SOAs align to local authority district (LAD) boundaries, including those that changed between 2003 and 2011, and also at the border between Scotland and England. 161 OAs and SOAs were modified because they were considered unsuitable for reporting statistics. The average population in an OA has increased from 297 in 2001 to 309 in 2011.

Boundaries are available clipped to the coastline, for mapping, as well as to extent of the realm, for geographic information systems and analysis. All OAs and SOAs have unique nine-character codes, in line with all statistical geographies we provide.

An upper layer super output area (USOA) will not be created for England as part of the 2011 Census OA hierarchy. Data Unit (Wales) has created a set of 2011 USOAs for Wales.

Boundaries are freely available under the terms of the Open Government Licence.

Scotland:

2011 OAs for Scotland were released in September 2013.

Northern Ireland:

In Northern Ireland, the 2001 OAs have been merged to produce new 2011 “small areas” that fit within SOAs. There are approx. 4,500 small areas.

Modification of Output Areas in England and Wales

Changes in OA and SOA boundaries for the 2011 Census have taken place when:

significant population change has occurred since the 2001 Census

LAD boundaries have changed between 2003 and 2011

OA boundaries have been realigned to the England/Scotland border, as should have happened for 2001

areas have been independently assessed as lacking social homogeneity when they were created for 2001

Redesigned OAs and SOAs:

do not align to ward and parish boundaries that have changed since 2003

do not necessarily align to real-world features

contain more than 100 persons and 40 households, even if they contain one or more communal establishments

A number of geography reference data products for electronic download, alongside the relevant stage of 2011 Census statistics release, are available. These include:

lookups between OAs, postcodes and a number of census output geographies

Thresholds used in Modification (England and Wales)

ONS runs a process to automatically modify those OAs and SOAs whose 2011 Census populations have significantly grown or declined since 2001. If OAs breached a specified upper population threshold (their populations became too large), they were split into two or more OAs using postcodes as building blocks.

Splits were applied where:

an OA population exceeded 625 people or 250 households

an LSOA population exceeded 3,000 people or 1,200 households

an MSOA population exceeded 15,000 people or 6,000 households

There may be exceptions where an area that was above the population threshold could not be split. Where splits occur, building blocks of postcodes were used to create two or more new OAs (constrained to the boundary of the original OAs from which they were created). The use of building blocks (postcodes) is consistent with the methodology applied in 2001, and enabled production of postcode to OA lookups.

Where OAs or SOAs breached a specified lower population threshold (their population became too small and is therefore potentially disclosive), they were merged with an adjacent OA or SOA.

Merges were applied where:

an OA population fell below 100 people or 40 households

an LSOA population fell below 1,000 people or 400 households

an MSOA population fell below 5,000 people or 2,000 households

Using splits and merges of the existing OA and SOA hierarchy, rather than a total redesign, allows better linkage and comparison between statistical outputs for the 2001 Census and 2011 Census.

5. Statistical wards, CAS wards and ST wards

Statistical Wards

In 2003, a National Statistics policy was introduced to minimise the statistical impact of frequent electoral ward boundary changes, particularly in England.

Under this policy any changes to English or Welsh electoral ward boundaries promulgated (laid down in statute) by the end of a calendar year, were implemented for statistical purposes on 1 April of the following year, irrespective of the year the actual change came into operation.

The wards resulting from this policy were known as 'statistical wards'.

So, for example, 2003 statistical wards were those that were promulgated by 31 December 2002.

In general they reflected actual electoral wards as at May 2003, but for 28 local authority districts (LAD) they also included boundary changes that were not operational until June 2004.

Therefore, for any given year statistical wards in some LADs were different to the statutory electoral wards because of the varying time lags between the promulgation and operation dates of boundary changes.

Please see the 'Further Information' section below for a list of those LADs subject to boundary change in May 2004, as promulgated by December 2002.

In 2006, a change to the policy was agreed by the National Statistics Geography Group (NSGG).

The 1 April implementation date is retained, but it now relates to those administrative and electoral areas that are statutorily operative on 31 December of the previous calendar year (that is, statutory electoral wards).

As such, statistical wards no longer exist – the last set produced was 2005 statistical wards.

Census Area Statistics (CAS) Wards

CAS wards were created for 2001 Census outputs, including those available on the Neighbourhood Statistics website.

In England and Wales they were identical to the 2003 statistical wards, except that 25 of the smallest (sub-threshold) wards were merged into 7 receiving wards to avoid the confidentiality risks of releasing data for very small areas.

This happened to those wards with fewer than 100 residents or 40 households (as at the 2001 Census).

There were a total of 8,850 CAS wards in England and Wales, 18 fewer than the total number of 2003 statistical wards.

Please see the 'Further Information' section below for details of those 2003 statistical wards that were merged to create the CAS wards.

Scotland also had CAS wards but these were created from best-fit 2001 Census Output Area (OA) aggregations to 2001 electoral wards.

There were 1,222 Scottish CAS wards, with a minimum size of 50 residents and 20 households.

Please note also that Scottish Census outputs used different ward codes to the ONS standard. See the 'Further Information' section below for this lookup file.

In Northern Ireland 2001 Census outputs used the 582 electoral wards in existence at Census Day.

There was no requirement to introduce specific CAS wards, as all electoral wards exceeded the 100 residents/40 households threshold.

However, as in Scotland, Northern Ireland 2001 Census outputs used different ward codes to the then ONS standard. Please see the 'Further Information' section below for this lookup file.

Standard Table (ST) Wards

ST wards were those for which the 2001 Census Standard Tables were available.

They were a further subset of the statistical wards such that those with fewer than 1,000 residents or 400 households were merged.

This was required to ensure the confidentiality of data in the Standard Tables.

In England and Wales a total of 113 of the 2003 statistical wards were involved in mergers to create the ST ward set.

Of the smallest (sub-threshold) wards, 81 were merged into 45 receiving wards, of which 13 were sub-threshold in their own right.

There were a total of 8,800 ST wards in England and Wales, 68 fewer than the total number of 2003 statistical wards.

Scotland's 1,176 ST wards had the same minimum-size thresholds but did not always correspond exactly with Scottish CAS ward boundaries – the NRS website provides more explanation.

In Northern Ireland it was decided that ward-level statistics would also be released for the 9 sub-threshold wards on the basis that the risk of disclosure would not be increased by doing so.

Accordingly there are no ST wards in Northern Ireland.

Note about Names of CAS and ST Wards

When two or more wards were merged to create CAS or ST wards in England and Wales, the name given to the new (enlarged) ward was that of the largest of its constituent wards (in terms of population).

For example, Cwm-y-Glo statistical ward (Gwynedd) was merged with the larger Bethel statistical ward to create the Bethel ST ward.

You must therefore be aware of which ward set you are using – in this case Bethel ST ward covers a much larger area and population than Bethel statistical ward.

6. Super Output Area (SOA)

Background

SOAs were designed to improve the reporting of small area statistics and are built up from groups of output areas (OA). Statistics for lower layer super output areas (LSOA) and middle layer super output areas (MSOA) were originally released in 2004 for England and Wales. Scotland also released statistics for data zones (DZ), that were equivalent to LSOAs, in 2004 and intermediate geographies (IG), that were equivalent to MSOAs, in 2005. Northern Ireland introduced LSOAs in 2005 but do not have an MSOA geography.

2011 Super Output Areas

Maintaining stability as far as possible was key for the 2011 Census. LSOAs and MSOAs created following the 2001 Census continue to exist unless a significant population change occurred between 2001 and 2011, and household minimum and maximum thresholds were breached. Simplistically, where populations have become too big, the LSOAs/MSOAs have been split into two or more areas; where populations have become too small, the LSOAs/MSOAs have been merged with an adjacent one. Responses to the Output Geography Consultation from December 2009 to March 2010 were also considered in the redesign of OAs and SOAs. Consequently, the total changes across the OA hierarchy were no more than 5% overall.

Population and household minimum and maximum thresholds for SOAs in England and Wales

SOAs in Northern Ireland have had minimal changes in three areas, but the total number of SOAs has not changed since the 2011 Census. Further details of Northern Ireland LSOAs can be found on the NISRA website.

Significant points of interest for the 2011 Census are that SOAs align to local authority district (LAD) boundaries including those that changed between 2003 and 2011, and also align at the border between Scotland and England.

Furthermore, the boundaries are available clipped to the coastline, for mapping, as well as to the extent of the realm, for geographic information systems and analysis. All SOAs have unique nine-character codes, in line with all statistical geographies we provide.

An upper layer super output area layer will not be created for England as part of the 2011 Census OA hierarchy.

2001 Super Output Areas

The total of 2001 LSOAs and MSOAs for the UK

Download this table

2001 SOAs were initially introduced for use on the Neighbourhood Statistics (NeSS) website, but later became the standard units for presenting local statistical information across National Statistics.

Local statistics were produced at electoral ward/division level before OAs and SOAs were introduced. This had drawbacks because electoral wards/divisions vary greatly in size, from fewer than 100 residents to more than 30,000.

This was not ideal for nationwide comparisons, and also meant that some data could not be released for smaller wards due to disclosure issues and the need to protect the confidentiality of individuals.

DZs and IGs in Scotland are smaller in population size than their LSOA and MSOA counterparts in England and Wales. DZs have a minimum population of 500 and IGs have a minimum population of 2,500.

In Northern Ireland, LSOAs have a population threshold of between 1,300 and 2,800.

SOAs: Frequently Asked Questions

7. Workplace Zone (WZ)

Workplace zones (WZ) are a new output geography for England and Wales that has been produced using workplace data from the 2011 Census. It may be extended to Scotland and Northern Ireland in 2015.

WZs are designed to supplement the output area (OA) and super output area (LSOA and MSOA) geographies that were introduced with the 2001 Census. OAs were originally created for the analysis of population statistics using residential population and household data. As a result, they are of limited use for workplace statistics as there is no consistency in the number of workers or businesses contained within an OA. OAs are designed to contain consistent numbers of persons, based on where they live, WZs are designed to contain consistent numbers of workers, based on where people work. This means that WZs are more suitable for disseminating workplace-based statistics and outputs.

WZs have been created by splitting and merging the 2011 OAs to produce a workplace geography that contains consistent numbers of workers. The WZs align to the existing OA hierarchy. They have been constrained to MSOA boundaries to provide consistency between the OA and WZ geographies, and to allow comparison of the 2001 and 2011 Census workplace outputs at the MSOA level.

8. Travel to Work Area

For those involved in labour market analysis and planning, it is useful to be able to use data for labour market areas.

To meet this need, labour market areas are defined to reflect areas where the bulk of the resident population also work within the same area.

Defining these areas requires the analysis of commuting patterns; we have worked with Newcastle University to apply a complex allocation process in order to define a set of travel to work areas (TTWA) for the whole of the UK.

The current criteria for defining TTWAs is that generally at least 75% of an area's resident workforce work in the area and at least 75% of the people who work in the area also live in the area. The area must also have a working population of at least 3,500. However, for areas with a working population in excess of 25,000, self-containment rates as low as 66.7% are accepted. TTWA boundaries are non-overlapping, are contiguous and cover the whole of the UK. TTWAs do cross national boundaries, although no account is taken of commuting between Northern Ireland and the Republic of Ireland.

The resulting pattern is that although the definitive minimum working population in a TTWA is 3,500, many areas are much larger – indeed, much of London and its surrounding area forms one TTWA.

2011 Travel to Work Areas

Please note that following the release of the 2011 TTWAs on 19 August 2015, a small number of TTWA geography codes have been revised. All the published documentation has been updated where applicable to reflect these changes.

The 2011 TTWAs number 228. They cover the whole of the UK and were defined in 2015 using 2011 Census commuting flow data, indicating home and workplace address. The TTWAs are based on aggregations of Lower Layer Super Output Areas (LSOA) in England and Wales, Data Zones (DZ) in Scotland, and Super Output Areas (SOA) in Northern Ireland, and in some cases span country borders (England/Wales and England/Scotland).

Supporting documentation including a map, guidance and information can be accessed in the links below:

2001 Travel to Work Areas

The 2001 TTWAs numbered 243. They covered the whole of the UK and were defined in 2007 using 2001 Census commuting flow data, indicating home and workplace address. The TTWAs were based on aggregations of LSOAs in England and Wales, DZs in Scotland, and SOAs in Northern Ireland, and in some cases spanned country borders (England/Wales and England/Scotland).

Supporting documentation including a map, guidance and information can be accessed in the links below: